Electric valve control system



1942- A. L. WHITELEY ETAL 2,269,967

ELECTRIC VALVE CONTROL SYSTEM 2 Sheets-Sheet 1' Filed Jan. 5, 1940nventors:

Austyn L Whiteley, k Bert-ram 6 Hi gins, by, 5 Their- Attorney;

Jan. 13, 1942. A. L. WHITELEY Em 2,269,967

ELECTRIC VALVE CONTROL SYSTEM Filed Jan. 5, 1940 2 Sheets-Sheet 2 Fig.2.

I TIME Fig.3

Inventors:

Aus'byn L Whiteley,

Bertram'fiHi gins, by fi a 7 Their" Attorney.

ply circuit to a load circuit.

Patented Jan. 13,

ELECTRIC vALvE CONTROL SYSTEM Austyn Ii. Whiteley and Bertram G.Higgins, Rugby, England; usignors'to General Electric Company, acorporation olNew York- Application January 5, 1940. Serial No. 312,520vIn Great Britain March 20, 1939 4 10 Claims.

Our invention relates to electric translating apparatus and moreparticularly to control systems for electric valve translating apparatusfor transmitting energy from an alternating current sup Electric valveapparatus has been found very eifective in applications where it isdesired to energize a load circuit, such as a welding circuit, from analternating current supply circuit during accurately determinableintervals of time. Due to the precision of operation afforded byelectric valve means, it is possible to initiate energization of theload circuit at a precise point during a cycleof voltage of thealternating current circuit and to transmit a predetermined value ofcurrent to the welding circuit during an accurately describedhereinafter, we provide new and improved control circuits for electricvalve apparatus whereby greater facility in the control of electricvalve apparatus is obtained.

It is an object of our invention to provide new and improved electricvalve translating apparatus.

It is another object of our invention to provide new and improvedcontrol systems for electric valve apparatus employedto energize a loadcircuit, such as a welding circuit, from an alternating current supplycircuit.

It is a further object of our invention to provide new and improvedcontrol systems for electric valve apparatus whereby current may betransmitted to a load circuit during definite intervals of time andwhereby the current transmitted to the load circuit during suchintervals is variable and accurately determinable.

It is a still further object of our invention to provide new andimproved electric valve timing circuits forcontrollingthe conductivityof electric valve translating apparatus.

Briefly stated, in the illustrated embodiment of our invention weprovide electric valve translating apparatus for interconnectinganalternating current supply circuit and a load circuit, and

-time; Another aspect of, our invention concerns the new and improvedelectric valve timing circuits for controlling the periods ofconductivity of the electric valve meansand for controlling the amountof current transmitted to. the load circuit during such intervals oftime.

Fbr a better understanding of our invention, reference may be had to thefollowing description, taken in connection with the accompanyingdrawings,'and its scope will be pointed out in the appended claims. Fig.1 diagrammatically illustrates an embodiment of our invention asapandilassociated with the electric valve means plied to a weldingsystem for energizing an alternating current welding circuit from analtemating current supply circuit, and Figs. 2 and 3 represent certainoperating characteristics thereof.

Referring now to Fig. 1 of the accompanying drawings, we havediagrammatically illustrated our invention as applied to a weldingsystem for energizing a load circuit, such as a welding circuit I; froman alternatingcurrent supply circuit 2 through a transformer 3 and apair of reversely connected electric valve means 4 and 5. If desired,one terminal of the primary winding of the transformer 3 maybe connectedto ground and one terminal of the alternating current supply circuit mayalso be connected to ground, as shown in the drawings. The electricvalve means 4 and 5 are preferably of the type employing an ionizablemedium, such as a gas or a vapor, and

each comprises an anode 6, a cathode such as a mercury pool cathode I,and may comprise makealive or immersion-ignitor control members 8 ofsemi-conducting material, such as boron-carbide or silicon-carbide,associated with the mercury pool cathodes. These control members requirethe transmission of a current of predetermined yalue therethrou'gh inorder to initiate an arc discharge between the anode and the cathode.

We provide a pair of excitation circuits 9 4 and 5, respectively, fortransmitting unidirectional current to the control members 8 thereof torender the valves conductive and hence to effectenergization of thewelding circuit I. Excita'tion circuits 9 and I0 include controlelectric valves or electric discharge devices H and i2, and I3 and M.respectively. These electric discharge devices are also preferably ofthe type employing an ionizable medium and'each comprises a controlgridIS. The excitation circuits 9 and I0 may be connected to be responsiveto the anode volt- :age of the associated electric valves 4 and 5, re-

spectively, and may be connected to the respective anodes throughcurrent limiting resistances l8, l1 and current protective means such asfuses l8 and I8, "respectively.

To render the electric discharge devices l3 and i 4 conductive, weprovide grid circuits 28 and 2|, respectively. These grid circuitsinclude means for impressing on the control grids i 5 thereof suitablebiasing potentials tendinsto maintain the electric discharge devicesnon-conducting. These biasing means may comprise transformers 22 and 23,respectively, and may be energized from any suitable source ofalternating current of' proper frequency and phase displacement,preferably having a phase displacement of 180 electrical degrees withrespect to the applied anode-cathode voltages of electric valves 4 and5, respectively. The transformers 22 and 23 may be energized from thealternating current circuit 2 through suitable phase shifting apparatus(not shown). A suitable source of substantially constant negativeunidirectional biasing potential may be employed in each of the gridcircuits 20 and 2| and may comprise a parallel connected capacitance 24and a resistance 25, both of which are connected in series relation,with a unidirectional conducting device 28 and whlchare energized fromtransformer 22 or 23.

Generally speaking, we provide, as explained hereinafter, new andimproved electric valve timing circuits for controlling theconductivities of the electric valves 4 and 5 so that these valvesconduct current during predetermined intervals of time, or, in otherwords, a predetermined number of cycles of the voltage of thealternating current circuit 2 to effect energization of the weldingcircuit I during a corresponding interval of time. In addition, weprovide a master timing circuit which generates a number of electricalquantities for determining independently separate or distinct intervalsof conduction by the electric valve means 4 and Band which operate inconjunction with current or heat control apparatus to transmit differentamounts of current to the welding circuit l during therespectlveintervals. The system is capable of responding to transmit differentamounts of current to the welding circuit during contiguous intervals inresponse to a single circuit controlling operation.

As a means for producing a number of electrical timing quantities, weprovide a master timing circuit 21. The timing circuit 21 may beenergized from the alternating current circuit 2 through a transformer28 and a biphase rectifier 29 comprising a transformer 30,, electricvalves 3|, a smoothing inductance 32, and a filter cirouit 33. Acontrollin contactor 34 may be connected in series relation with thepositive terminal of the rectifier 29 and a suitable poten-.

tiometer comprising a resistance 35 may be connected across the outputterminals of the rectifier 29. In order to produce a number ofelectrical timing quantities, we provide in parallel a pair of electricpaths comprising resistances 36 and 31 and a capacitance 38, andresistances 39 and 48 and a capacitance 4|. These paths circuits areconnected to the positive terminal of the output circuit of rectifier 29and are connected to be charged from the output circuit through asuitable electric discharge device 42 preferably of the type employingan ionizable medium, such as a gas or a vapor, and having a control gridSuitable initiating switches 44 and 45 are connected in series relationwith the anodecathode circuit of the electric discharge device 42 andserve to initiate the charge of the capacitances 38 and 4| from thedirect current output .circuits from attaining dangerously high values.

A capacitance 49 is connected across the cathode and the grid ofelectric discharge device 42 to absorb extraneous transient voltages. Inorder to render the electric discharge device 42 conductive at a precisetime during a cycle of voltage circuit 2, we employ a transformer 50preferably of the type designed to produce a voltage of peaked wave formand which is connected in circuit with the control grid 43. A currentlimiting resistance 5| may be connected in series relation with the grid43. Transformer 59, if desired, may be connected to the alternatingcurrent circuit 2 through a suitable phase shifting device (not shown).This phase shifting device, of course, may be adjusted to adjust orcontrol the time during the cycles of voltage at which the electricdischarge device 42 is rendered conductive.

Switches 52 and 53 may be connected across the resistances 39 and 40,respectively, to control the time constants of the charging circuit forcapacitances 38 and 4| and hence to control the duration of theelectrical timing quantity. In this manner, it is possible to obtainready control of the periods of conduction by the electric valve means 4and 5.

To produce trains of cycles of alternating current in response to theelectrical timing quantities generated by the master timing circuit 21,we provide control circuits 54 and 55 which are energized from asuitable source of alternating current correlated in phase and frequencywith respect to the voltage of circuit 2. Of course, the controlcircuits 54 and 55 may be connected to circuit 2 by means of aninsulating transformer 58 and through a control switch 55', if desired.

Control circuit 54 comprises a pair of electric discharge devices 51 and58 preferably arranged in a leading and following relationship,respectively. The period of conductivity of the electric dischargedevices 51 and 58 is determined by the electrical timing quantityderived from the master timing means 21 through conductor 59 and avoltage divider 50 connected to the main voltage divider of the mastertiming means. A switch 6| may be employed to adjust or vary the intervalof time during which the electric valve 51 is rendered conductive. Thecurrent limiting resistance 52 and the transient absorbing capacitance63 may be connected in the grid circuit of the electric discharge device51. The electric discharge device 58 is arranged to follow the dischargedevice 51, that is, to be rendered conductive during the following halfcycle after each half cycle of conduction by the electric dischargedevice 51 since the devices are arranged inversely in parallel.Furthermore, the electric discharge devices 51 and 58 are arranged toconduct current from the supply circuit 2 through transformer 28 andthrough an inductance 64. Since the discharge devices 51 and 58 arearranged inversely in parallel, alternating current will be transmittedthrough the inductance 64 for a number of cycles determined by thetiming quantity derived from the master timing means 21. A suitablesource of biasing potential; such as an alternating biasing potentialpreferably displaced 180 electrical degrees with respect to theanode-cathode voltage of discharge device 58, is impressed on its gridby mean of a transformer 95. A self-biasing circuit comprising aresistance 89 and a capacitance 91 is also employed. Superimposed on thetwo biasing potentials there is also provided a voltage sufficient toovercome the eifect of the biasing potentials to render the dischargedevice 59 conductive due to the transmission of current by the dischargedevice 51. This control voltage may be obtained by mean of a transformer'68 which isconnected to be energized in response to the voltageappearing across the inductance 64. A resistance 69 of relatively highohmic value is connected across the inductance 64 to maintain thevoltage variations across the terminals thereof within a reasonablerange of values. In order to impress on the control grids I of theelectric discharg devices I2 and I3 trains of half cycles or cycles ofalternating voltage corresponding in length to one of the electricaltiming quantities generated by circuit 21, we provide a transformerenergized through a resistance 1i and having a pair of secondarywindings 12 and 19. Secondary winding 12 is connected to grid I5 ofdischarge device l2 through resistances 14 and 15, and secondary winding13 is connected to grid I5 of discharge device I3 through resistances 10and 11.

Control circuit is substantially similar to control circuit 54 and.transmits trains of .half

cycles or cycles of alternating voltage *corresponding to a differentelectrical timing quantity derived from the master timing circuit 21through a circuit including conductor 18, a voltage divider 19 and aswitch 19. The control circuit 55 includes a pair of reversely connectedelectric valve means and 8| and impresses upon a transformer 82 trainsof half cycles or cycles of voltage to render the-electric dischargedevices I I and I4 conductive during a period of time corresponding tothe timing quantity derived from the'master timing circuit 21. Thetransformer 82 comprises secondary windings 83 and 84, the former ofwhich is'connected to grid I5 of electric discharge device II throughresistances B5 and 86, and the latter of which is connected to grid I5of discharge device I4 through resistances 81 and 88. The controlcircuit 55 is connected to render discharge devices II and and henceelectric valves 4 and 5 conductive during a predetermined interval oftime by impressing on the control grids I5 of the discharge devices atrain of cycles of alternating potential to render the electric valves 4and 5 conductive during a corresponding number of cycles. In controlcircuit 55 the discharge devices'80 and 9| are arranged in leadingandfollowing relationship to transmit alternating current through theinductance 99. An alternating biasing or half-off potential may beprovided by transformer 90, and a voltage for rendering the circuits 29and alternatingvoltages of peaked wave form to determine the timeduringthe cycles of voltage applied to electric valves land-l at whichthe electric valves are rendered conductive Considering control circuit92 in particular, this circuit comprisesa suit-able means such as asaturable inductive transformer 94 having a primary winding 95 and apair of secondary windings 99 and 91. A suitable capacitance 99 may beconnected across the primary winding 95 of the transformer 94 in orderto compensate for the magnetizing current required -thereby. In order tointroducejthe alternating voltage of peaked waveform generated bytransformer 94 into the grid circuits 20 hand 2|, winding and 91 areconnected across resistances and 91, respectively, so that the voltagesof peaked wave form effectively act in series relation with respect tothe other aforementioned control potentials of these grid circuits. .Tocontrol or adjust the phaseof the alternating voltage of peaked waveform generat ed in windings 96 and 91, we provide suitable phaseshifting circuits preferably of the static impedance type and which maycomprise a capacitance 99, an inductance I00, a resistance IN, andswitches I02 and I03. Other suitable variable resistances I04 and I05may also be employed to adjust, control or preselect the phase positionof the voltages of peaked wave form generated in windings 9B and 91. Itwill be noted that the left-hand terminal of the primary winding oftransformer 94 is connected to the mid-connection of the secondarywinding of transformer 28, and that by operation of the switch I03 thereis provided means for connecting the primary winding 95 selectively toalternating voltages electrical degrees displace from each other.

Control circuit 93 is similar in all substantial respects to circuit92.- This circuit comprises a transformer, I06, preferably of the typedesigned to produce a voltageof peaked wave fomnand comprises secondarywindings I01 and I08 connected across resistances 14 and 15 in gridcircuits 20 and 2|, respectively. In other words, the alternatingvoltages of peaked wave form generated in secondary windings I01 andI09. deterdischarge device 9| conductive inresponse to In order tocontrol the amount of current transmitted to the welding. circuit Iduring the intervals of time established'by the master tim- Other i minethe time during the half cycles of voltage of circuit 2 at whichthe'electric discharge devices I2 and I3 are rendered conductive,and

hence control the amount of current transmitted to the welding circuit Iby the electric 'valve means 4 and 5 during one of the intervals of'master timer .21. "'In other details, the control circuit 93 is exactlythe same as circuit 92. Clrelectrodes at a definite time during the'welding operation. We employ a circuit 199 for operata control circuitI10 which, in turn. controls apparatus forvaryin'gthe pressure exertedby the welding electrodes. The circuit I09 comprises a transformer III,a suitable rectifying device H2 and a filter circuit III comprisingcapacitances Ill and an inductance III. A suitable voltage dividerincluding resistances II! and H1 may be connected across the directcurrent terminals H and H9. In order to effect energization of thecontrol circuit H0 at the desired time during the welding cycle andhence to increase the pressure between the welding electrodes at thedesired time, we provide in circuit I09 an electric discharge device I20for energizing the actuating coil I2I of a relay I22. The electricdischarge device I20 is preferably of the gaseous type having itsanode-cathode circuit connected in series relation with the actuatingcoil I2I of relay I22. A capacitance I22 and a resistance I24 may beconnected across the actuating coil I2I in order to transmit asubstantially unidirectional current through the actuating coil I2Iduring the interval of energization thereof. The actuating coil I2I isconnected to be energized from the alternating current circuit 2 throughtransformer 29 and a current limiting resistance I25.

The timing of the relay I22 is effected by means of a capacitance I29which is connected in series relation with the grid I21 or dischargedevice I20. Normally, the discharge device I20 is maintainednon-conductive by virtue of the connection of the grid I21 to a pointmorenegative in potential than its cathode. If desired, this effect maybe obtained by connecting the grid I21 to a relatively negative point ina voltage divider I20 through a resistance I29. A timing switch I30 mayalso be connected in series relation with the grid I21 and the voltagedivider I20 in order to initiate the operation. A discharge resistanceI9I is connected across the terminals of the capacitance I29 to resetthe circuit.

Certain features of the control wherein the intensity of the currenttransmitted to the welding circuit and the pressure are'correlated arebeing'claimed in our copending divisional application Serial No.376,542, filed January 29, 1941, and which is assigned to the assigneeof the present application.

Circuit IIO may be connected to suitable means for increasing thepressure exerted by the welding electrodes on the work. As adiagraminatlc illustration of such apparatus, I have chosen to show acoil I32 which is connected to be energized from a suitable source ofcurrent such as a battery I33 to increase the pressure exerted by theelectrodes on the work at a definite time during the welding cycle. Forexample, the switch I30 may be arrangedto be operated simultaneouslywith switches 44 and 45 to initiate the operation of circuit I09simultaneously with the initiation of the operation of the timingcircuit 21. The potentiometer I20 may be adjusted so that the pressureis increased at the beginning of the second period of energization ofthe welding circuit I, that is at the beginning of the period of lowercurrent intensity. It will be understood that the means shown forincreasing the pressure is merely a diagrammatical illustration.

The operation of the embodiment of our invention shown in Fig. 1 will beexplained by considering the system when it is operating to transmitalternating current to the welding circuit I during predeterminedintervals of time, and in which the values oi the current transmitted tothe welding circuit I are different throughout the respective intervalsof energization thereof. As will be well understood by those skilled inthe art, the

magnitude of the current transmitted to the welding circuit I will beincreased in value as the time of initiation of arc discharges in theelectric valve means 4 and 5 is advanced from a lagging position to aposition more nearly in phase with the zero value at the beginning ofthe positive halt cycles of anode-cathode voltage. Conversely, as thetime 01' initiation or the arc discharges is retarded, the magnitude ofthe current transmitted to the welding circuit I is decreased in value.

Master timing circuit 21 generates two electrical timing quantities ofdiiierent duration to control circuits 54 and 55 and thereby producestwo diflerent trains of cycles of 'control voltage or current ofdifferent duration. The manner in which the timing circuit 21 generatesthese electrical timing quantities will now be explained. With thecontrol switches 40 and 45 in the positions shown, the capacitances 38and I are short circuited and when the switches 44 and 45 are moved tothe left-hand position the electric discharge device 42 will beconnected in circuit with the capacitances 38 and 4| to initiate thecharging of these capacitances from the output circuit of rectifier 29.The exact time at which the charging of capacitances 38 and H isinitiated is determined by the Setting of the peak voltage introduced inthe grid circuit of discharge device 42 by means of the transformer 50.The time constants of the charging circuits for capacitances 38 and 4|,in conjunction with voltage dividers 60 and 19, determine the durationof the timing quantities generated by circuit 21. In order to obtaintiming quantities of diiierent duration, the resistances 35 and 91, and39 and 40 are adjusted to have different values.

The operation of the master timing circuit 21 may be explained morefully by referring to the operating characteristics shown in Fig. 2. Theeffect of the adjustment of the voltage dividers 50 and 19 determinesthe period of time during which the electric discharge devices 51 andII, and and 9| in circuits 5| and 55, respectively, are renderedconductive. Curves N and M show the relation between the time requiredto render discharge devices 51 and 58 non-conductive and the setting ofthe voltage divider 60. Curve M shows the relationship existing whentheswitch 5| is in the position indicated in Fig. 1, while curve N showsthe relationship existing when the switch BI is moved to the left-handposition. With a certain setting of the slider or contact of voltagedivider 60, the duration of the final portion of the welding period maybe adjusted to the horizontal line 0, while on moving the contactdownwardly the duration of the welding period may be reduced to the timeindicated by the horizontal line P. On changing the switch N to thesecond position, that is to the left-hand position, it is obvious thatthe two time periods may, by equivalent settings of the controls, be

considerably increased since the lines 0 and P will be extended to cutcurve N.

Control circuits 54 and 55 transmit to the primary windings oftransformers 10 and 02, respectively, trains of cycles of alternatingvoltage, the duration of the trains and the number of cycles in therespective trains being different because of the difference in length ofthe electrical timing quantities derived from circuit 21. Electricdischarge devices 51, 58 and 90, 0I are rendered conductive by thetransmission of current through resistances 96, 31 and 39, 40.respectively, upon the charging of capacitances I0 and 4| through theelectric discharge device 42. During the charging of the capacitances 39and H the common'junctures of the resistances and the associatedcap'acitances are lowered to effect a corresponding lowering inpotential of the cathodes of electric discharge devices 51 and 90 andthereby render these discharge devices conductive by establishing asuflicient potential difl'erence between the respective control gridsand the associated cathodes. The electric discharge devices 51 and 58,and 90 and BI in circuits 54 and 55, respectively, conduct current.altemately to transmit these cycles of alternating current totransformers I and 92.

Electric valves 4 and normally are non-conductive and are renderedconductive by the transmission of current to the *immersion-ignitorcontrol members 8. The electric valve 4, for example, is renderedconductive when either electric discharge device II or I2 is renderedconductive, and likewise the electric valve 5 is rendered conductivewhen either electric discharge device' I3 or I4 ,is rendered conductive.To explain the operation' of the system, it will be assumed that thecontrol circuit 55 transmits a smaller number of cycles of alternatingcurrent to the transformer 82 than the number of cycles transmitted totransformer by circuit 54. Furthermore, it will be assumed that controlcircuit 92 is adjusted so that the alternating voltages of peaked waveform which determine the time at which the electric discharge devices'II and I4 are rendered conductive is advancedsubstantially with respectto the volt-' ages of peaked wave form produced by secondary windingsI01 and I08 which determine the time at which discharge devices I2 andI3 are rendered conductive. In other words, during the first interval ofconduction by electric valves 4 and 5, the length of the period ofenergization of the welding circuit I is determined by the controlcircuit 55 through discharge devices II and I4, and the length oi. thesecond period of energization of the welding circuit I is detemiined bythe control circuit 54 acting upon valves I2 and I3. It will beunderstood that during the first interval of energization of the weldingcircuit I, both control circuits 54 and 55 tend to render the electricvalves 4 and 5 conductive, but circuit 54 is ineffective due to the factthat its associated heat control circuit 93 is adjusted to produce apeak voltage lagging the peak voltage of circuit 92. In other words, thefirst discharge device of discharge devices II, I2 and I3, I4 to berendered conductive effects initiation of an arc discharge in theassociated principal electric valves 4 and 5, respectively, andimmediately upon such action the anode-cathode voltage 'decays to avalue substantially equal to the arc drop of the valve and the seconddischarge device to be rendered conductive is ineifective. However,since the trains of impulses transmitted by circuit 54 exceed in numberthose transmitted by circuit 55, circuit 54 and circuit 93 assumecontrol during an interval of time equal to the difference in length ofthe two electrical timing quantitles derived from circuit 21. After theexpiration of the timing quantity derived from voltage divider I9,control circuit 55 ceases to operate I. Of course, when circuit 54 takesover control, electric valves 4 and 3 are rendered conductive by meansof electric discharge devices I2 and I3.

The manner in which the grid circuits 20 and 2I control electric valves4 and 5 to transmit different amounts oi current to the welding circuitI during predetermined diflerent intervals may be more fully explainedby considering the operating characteristics shown in Fig. 3. Curve A ofFig. 3 represents one-half cycle of anode voltage applied to one of thecontrol valves, such as electric discharge device I2. Curve B shows thecorresponding critical grid voltage for that discharge device; and curveC represents-the blocking or biasing potential impressed thereon bymeans of transformer 22. Superimposed on the biasing potential is thereleasing or firing potential represented by curve D which is derivedfrom circuit 54 through secondary winding I2 of transformer I0. Curve Erepresents the resultant of the two curves C and D on which issuperimposed the peaked voltage derived from secondary winding I01 oftransformer I06 in circuit 93. It is evident that the amplitude of thevoltage of peaked wave form must be such that it is capable ofdiminishing the negative bias on the grid of the discharge device I2 toa value above the critical voltage to enable the valve to becomeconductive at the desired instant, and it must therefore have anamplitude at least equal to the maximum diiiference between the curves Band E. With such an amplitude, however, if the peak voltage is advancedto a position indicated in the dotted line at G to give a large averagecurrent flow through. the valve, the peak voltage will be able, even inthe absence of the releasing voltage, to render the valve conductive asindicated. As a result, there is a loss of control and the electricdischarge devices will remain conducting after the end of the properconductive periods. To avoid this difiiculty, the capacitance 24 andresistance 25 inserted in the grid circuits 20 and 2 I, in conjunctionwith the rectiflers 26, produce a substantially unidirectional bias. Thepresence 1 nitude of the current transmitted to the load circuit duringthe various intervals of energization may be adjusted or controlled bymeans of the heat control circuits 92 or 93.

If desired, the circuit I09 may be employed to increase the pressureexerted by the welding electrodes upon the work during a predeterminedtime of the welding cycle. For example, the switch I30 may be operatedsimultaneously with switches 44 and 45 so that the relay I22 is operatedat the beginning of the second period of energization of the weldingcircuit I. The relay I22 is operated to close its contacts to effectenergization of the coil I32 at the beginning of the second or lowcurrent intensity interval of energization. Of course, the circuit I09may be adjusted by means of the voltage divider I28 to determine thetime during the welding cycle at which this increased pressure isapplied.

While we have shown and described our invention as applied to particularsystems of connections and as embodying various devices diagrammaticallyshown, it will be obvious to those skilled in the art that changes andmodifications may be made without departing from our invention, and we,therefore, aim in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In combination, an alternating current supply circuit, a loadcircuit, electric translating apparatus connected between said circuitsand comprising a pair of reversely connected electric valve means eachhaving a control member for controlling the conductivity thereof, a pairof excitation circuits each associated with a diilferent one of saidelectric valve means for energizing the control member thereof and eachcomprising a pair of electric discharge devices, timing means forcontrolling one of said discharge devices in each of said excitationcircuits to render said electric valve means conductive during apredetermined number of cycles of voltage of said alternating currentcircuit, and a second timing means connected to the other electricdischarge device in each of said excitation circuits to render saidelectric valve means conductive during a second predetermined number ofcycles of volt age of said alternating current circuit.

2. In combination, an alternating current supply circuit, a loadcircuit, electric translating apparatus connected between said circuitsand comprising a pair of reversely connected electric valve means eachhaving an anode, a cathode and a control member for controlling theconductivity thereof, a pair of excitation circuits each associated witha different one of said electric valve means and each connected betweenthe anode and the control member of the associated electric valve meansand each comprising a pair of parallel connected electric dischargedevices, said electric discharge devices each having a control grid,timing means connected to the control grid of one discharge device ineach of said excitation circuits to render said electric valve meansconductive during a predetermined interval of time, and a second timingmeans connected tothe control grids of the other discharge device ineach of said excitation circuits to render said electric valve meansconductive during a second predetermined interval of time.

3. In combination, an alternating current supply circuit, a loadcircuit, electric translating apparatus connected between said circuitsand comprising a pair of reversely connected electric valve means eachhaving a control member for rendering said electric valve meansconductive, a pair of excitation circuits each associated with adifferent one of said electric valve means for transmitting energizingcurrent to the control member thereof and each comprising a pair ofelectric discharge devices, said discharge devices each comprising acontrol grid, timing means for energizing the grid of one electricdischarge device in each of said excitation circuits to effectenergization of said load circuit during a predetermined interval oftime, a second timing means for energizing the grids of the otherelectric discharge devices in said excitation circuits to effectenergization of said load circuit for a second interval of time, andmeans connected to said grids to control the amount of power transmittedto said load circuit during said intervals.

4. In combination, an alternating current supply circuit, a loadcircuit, electric translating apparatus connected between said circuitsand comprising electric valve means having an anode, a cathode and acontrol member for controlling the conductivity thereof, an excitationcircuit for said control member comprising a pair of electric dischargedevices connected to said control membet and each comprising a controlgrid, a timing means for producing two electrical quantities, and a pairof control circuits controlled by said timing means and each beingconnected to the grid of a different one of said electric dischargedevices to energize said control member to effect energization of saidload circuit during intervals of time established by said electricalquantities.

5. In combination, an alternating current supply circuit, a loadcircuit, electric translating apparatus connected between said circuitsand comprising electric valve means having an anode, a cathode and acontrol member for controllin the conductivity thereof, an excitationcircuit for said control member comprising a pair of parallel connectedelectric discharge devices connected between said control member and theanode of said electric valve means, said electric discharge devicesbeing connected between said control member and the anode of saidelectric valve means, each of said electric discharge devices having acontrol grid, a timing means for producing two electrical quantities,and a pair of control circuits controlled by said timing means and eachbeing connected to the grid of a different one of said electricdischarge devices to energize said control member to render saidelectric valve means conductive during intervals of time established bysaid electrical quantities.

6. In combination, an alternating current supply circuit, a loadcircuit, electric translating apparatus connected between said circuitsand com prising an electric valve means having an anode, a cathode andan immersion-ignitor control member, an excitation circuit connectedbetween said anode and said control member and comprising a pair ofcontrol electric valves connected in parallel relation and each having acontrol grid to render the "valve conductive and to effect energizationof said control member from said alternating current circuit, and a pairof timing means each connected to the control grid of a different one ofsaid control electric valves to transmit different amounts of current tosaid load circuit during different predetermined intervals.

7. In combination, an alternating current supply circuit, a loadcircuit, electric translating apparatus connected between said circuitsand comprising electric valve means having an anode, a cathode and acontrol member, an excitation circuit for energizing said control memberand comprising a pair of electric discharge devices, a master timingcircuit for generating two electrical timing quantities of differentduration, a con trol circuit responsive to one of said two timingquantities for controlling one of said discharge devices to render itconductive during a predetermined number of cycles of voltage of saidalternating current circuit, and a second circuit responsive to theother timing quantity to control the other discharge device to render itconductive during a second interval of time.

8. In combination, an alternating current supply circuit, a loadcircuit, electric translating apparatus connected between said circuitsand comprising electric valve means having an anode, a cathode and animmersion-ignitor control memher, an excitationcircuit connected betweensaid anode and said control member and comprising a pair of controlelectric valves connected in parallel relation for transmittingunidirectional current to said control member, said control electricvalves each having a grid for controlling the conductivity thereof, amaster timing circuit for producing two electrical timing quantities ofdifferent duration, a control circuit responsive to control electricvalves a predetermined number of cycles 01' alternating voltagedetermined by the said one timing quantity to effect energization ofsaid load circuit for a corresponding interval of time, and a secondcontrol circuit responsive to the other electrical timing quantity andcomprising a pair of reversely connected electric discharge devicesenergized from said alternating current circuit for transmitting to thegrid of the other control electric valve a predetermined number ofcycles .of voltage determined by said other timing quantity to eflectenergization of said load circuit for a period of time determined by thedifference in length of said timing quantities.

9. In combination, a source of direct current, a pair of electric pathseach comprising a serially connected resistance and a capacitance, anelectric discharge device connected in series relation with said pathsand comprising an anode, a

cathode and a control grid, means tending to maintain said dischargedevice non-conductive, means connected to said grid tending to rendersaid discharge device conductive, and means connected in theanode-cathode circuit of said discharge device and in series relationwith said 10. In combination, an alternating current circult, a sourceof direct current, a pair of electric paths each comprising in seriesrelation a resistance and a capacitance, means for charging thecapacitances from said source of direct current and including anelectricdischarge device having an anode, a cathode and a control grid, avoltage divider connected across said source of direct current, meansfor impressing on said control grid a negative biasing voltage derivedfrom said voltage divider tending to maintain said discharge devicenon-conductive, means for impressing on said grid a voltage to rendersaid discharge device conductive at a predetermined time during a cycleof voltage of said alternating current circuit, and means connected inseries relation with the anode-cathode circuit of said discharge deviceand said electric paths for initiating the charge of the capacitancesfrom said source of direct current to produce two electrical timingquantities the duration of which is determined, by the time-constants ofthe'cir- AUSTYN L. WHITELEY. BERTRAM G. HIGGINS.

